Field of the Invention
The invention relates to a method and a radio station for data transmission via a radio interface in a radio communications system, in particular in a mobile radio network.
In radio communications systems, information (for example voice, video information or other data) is transmitted using electromagnetic waves. The electromagnetic waves are in this case transmitted at carrier frequencies that are in the system for mobile communication (GSM), the carrier frequencies are in the region of 900 MHz. For future radio communications systems, for example the universal mobile telecommunication system (UMTS) or other 3rd generation systems, frequencies are provided in the frequency band around 2000 MHz.
The transmitted electromagnetic waves are attenuated by losses due to reflection, refraction and transmission losses due to the earth""s curvature and the like. This results in a reduction in the reception power that is available at the receiving radio station. This attenuation is position-dependent and, for moving radio stations, time-dependent as well.
Between a transmitting and a receiving radio station, there is a radio interface via which data transmission takes place using electromagnetic waves. Published, Non-Prosecuted German Patent Application DE 195 49 148 A1 discloses a radio communications system which uses code division multiple access (CDMA) subscriber separation, with the radio interface also having time-division multiplex access (TDMA) subscriber separation. Such a radio interface is also known from a reference by Jung et al., titled xe2x80x9cA Joint Detection CDMA Mobile Radio System Concept Developed Within COST 231xe2x80x9d, IEEE, Proceedings of the vehicular technology conference, Chicago, Jul. 25-28, 1995, Volume 1, pages 469-473. A joint detection (JD) method is used at the receiving end in order to use knowledge of the spread codes of a plurality of subscribers to improve the detection of the transmitted data. In this case, it is known that a connection via the radio interface can be assigned at least two data channels, in which case each data channel can be distinguished by an individual spread code.
It is known from the GSM mobile radio network for transmitted data to be transmitted as radio blocks (bursts) within time slots, with midambles with known symbols being transmitted within a burst. The midambles may be used as training sequences in order to tune the radio station at the receiving end. The receiving radio station uses the midambles to estimate the channel impulse responses for various transmission channels. The length of the midambles is permanently defined, irrespective of the traffic conditions. Published, European Patent Application EP 0 615 352 A1 discloses a variably adjustable length for midambles in a TDMA system, in order to improve the channel estimation.
The number of channel impulse responses that can be estimated jointly represents a capacity-limiting factor for such radio communications systems. Since the number of symbols in the midambles is finite and a channel impulse response cannot be infinitely short, the number of channel impulse responses which can be estimated jointly is limited, as is the number of data channels transmitted jointly via the radio interface, as well.
It is accordingly an object of the invention to provide a method and a radio station for data transmission that overcome the above-mentioned disadvantages of the prior art methods and devices of this general type, which make better use of the radio resources of the radio interface.
According to the invention, in the method for data transmission in a radio communications system, a radio interface is subdivided into time slots for transmission of bursts. In this case, in one time slot, the data are transmitted in data channels, in which case the data channels can be distinguished by an individual spread code. A finite burst containing data symbols and at least one midamble with known symbols is used for data transmission in a data channel. At least one parameter is determined for the traffic conditions of the radio interface, and a ratio of a length of the midamble and a data part with data symbols is adjusted depending on the traffic conditions.
The limit on the number of channel impulse responses which can be estimated, and thus the limit on the number of connections which can be transmitted in a time slot can thus be overcome by lengthening the midamble. If the midamble is lengthened, then a greater number of connections can be transmitted. On the other hand, the midamble length can be shortened if there are only a small number of connections per time slot, so that a greater proportion of the time slot can be used for transmission of the data symbols. The capability to adjust the midamble length also applies to bursts within data channels of one connection type (wanted information, signaling information, organization information, access bursts).
According to advantageous developments of the invention, one parameter for the traffic conditions is:
the number of connections in the time slot, and/or
a terrain classification for a radio cell, and/or
the transmission quality in the time slot.
The number of traffic conditions per time slot, the present number and/or the desired number, takes account of the number of channel impulse responses which can be estimated.
The terrain classification takes account of the special features of individual radio cells. For example, greater scatter in the signal propagation times on different propagation paths can be observed in mountainous areas or fjords, as a result of which a long channel impulse response can be estimated. If the number of connections is constant, the midamble can be lengthened. On the other hand, in flat radio cells with few buildings, short channel impulse responses and thus short midambles can be used. The terrain classification may be predetermined (by the network plan), or may be derived from the present radio interface measured values.
The transmission quality, for example the bit error rate or the like, may be used as a parameter for estimating the quality of the channel estimation. If the previous length of the estimated channel impulse response is not sufficient, then this leads to poorer data detection. This can be counteracted by appropriately varying the ratio of the lengths of the midamble and data part.
If the midamble length is dynamically matched to the number of connections in the time slot and to the length of the channel impulse response to be estimated, then, on average, the spectral efficiency of the radio interface is improved.
According to a further advantageous refinement of the invention, the ratio of the length of the midamble and the data part with data symbols is adjusted as a function of time. Therefore, the midamble length is adapted, based on the present and/or desired traffic conditions at the radio interface. The structure of the burst is thus matched to the traffic conditions, without any major delay. This control can be carried out by a base station or by other network components.
The ratio of the length of the midamble and the data part with data symbols is alternatively or additionally adjusted for individual radio cells and/or for individual time slots. The traffic conditions fluctuate from radio cell to radio cell and from time slot to time slot, so that the flexibility of the radio communications system is improved if matching is carried out on an individual basis rather than across the network.
It is also within the scope of the invention for the midambles used in a time slot to be derived from a common midamble basic code. This allows the midambles to be produced particularly easily at the transmitting and receiving ends, and channel estimation to be carried out jointly for all the connections whose midambles have been derived from a common midamble basic code.
It is advantageous to assign a plurality of data channels to one connection, with the number of midambles that are used being less than the number of data channels. This reduces the channel estimation complexity. In addition, the number of possible data channels per time slot is increased, since a plurality of data channels use the same midamble, and the capacity-limiting influence of channel estimation has no effect on the data channels. It is likewise within the scope of the invention for the data channels with different midamble lengths to have different data rates. The different data rates may occur by varying the proportion of data symbols per time slot. Then, by way of example, voice transmission may be continued with a constant quality by switching to a so-called half-rate mode.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and a radio station for data transmission, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.